Flash memory is a form of non-volatile memory that can be electrically erased and reprogrammed. It is incorporated into a wide range of consumer products, such as digital audio players, digital cameras and mobile phones. However, all flash memories suffer from the disadvantage that they can only withstand a finite number of erase cycles. With the die area of flash memories becoming larger all the time, the statistical probability of any given bit in the memory becoming damaged increases.
For bulk storage applications, the popular choice is NAND flash because of the increased data density compared with NOR flash. The chief drawback with using NAND flash is that the individual bits of bytes cannot be randomly accessed. Instead, the device is arranged like a hard disc drive into 512 byte sectors. When a flash IC is manufactured and tested, it is expected that some of these sectors will be damaged due to the process and so extra sectors are available to replace those lost to general semiconductor yield issues. Often there is a complex controller that makes this process invisible to the outside, and typically there are 2% extra sectors available for this. A typical NAND flash sector can be reprogrammed about 10000 times.
Because of the inherent limited endurance of flash memories, many manufacturers put some simple error correction into the memory. Typically they use Hamming codes and increase the sector size by a further 16 bytes to accommodate the error correction overhead, but this data space is not available to the outside system. All of these techniques require extra die area on the flash device to perform these functions.
Using the error correction, the flash memory can correct only one bit in one sector (1 bit in 4096 bits) and detect 2 bits in error per sector. Flash manufacturers claim that this is sufficient for most purposes; however, some filing systems can cause the level of damage sustained by certain sectors to be greatly increased, causing the product to fail in a short period of time.
Filing systems, such as FAT16 and FAT32, save two copies of a table that is used to tell the host processor where everything is stored on the device. Every time any part of the bulk memory is changed, the filing system causes the two copies of this essential data to be re-written. In NAND flash memory, a single location or byte cannot be individually erased and so an entire block (covering several sectors) must be formatted and re-written each time part of the bulk memory is changed. This causes premature failures to many devices such as thumb drives.
One solution to this problem is to ensure that certain frequently accessed data items are not written back to the same area of the flash memory but rotate around the memory to spread the “wear and tear” over the entire device.
Clearly, manufacturers do not want to be constantly increasing the level of in-built sophistication of flash drives as this increases the cost without necessarily giving a perceived benefit to the user. Flash manufacturers are also unlikely to want to make a big issue about the inherent unreliability of their products.
One problem faced by consumer audio manufacturers is that their costs are increasingly linked to commodity memory market pricing. As the popularity of digital media grows, so this trend will increase. However, end customers do not allow their audio suppliers to factor this memory price fluctuation into their buy-price (as is the case in the PC market) and this leaves the audio suppliers exposed to the fickle whims and trends of the memory market.
The consumer audio industry has sought ways of overcoming this and recycling of memory has become commonplace. DRAM, another commodity memory product, is frequently salvaged from old SIMMS and often at a fraction of the ambient market prices. With a revolution in NAND flash demand from the audio electronics industry poised to happen, it seems likely that this type of memory product will also be targeted by component recycling companies.
Recycled flash memory will be characterized by a number of factors: (i) older technology and (ii) higher probability of defective sectors. Any flash controller entering the market should be adapted to deal with these disadvantages so that it can be used with recycled flash memory.